This changes configlet to pass a subcommand.
For now, we've released a version of configlet which handles both the old command:
configlet path/to/track
as well as the new command:
configlet lint path/to/track
This will let us update all the travis files to include the subcommand before we
release the version of configlet that requires the subcommand.
* Update exercises to elm-test 2.0
* Update docs to mention `elm-test` again
* Update .travis.yml to the correct version of elm-test
* Conform to the `<| \() ->` convention
